1. Field of the Invention
This invention relates to the art of multilayer wiring and more particularly, to an insulative ceramic composition useful as a material for multilayer wiring boards.
2. Description of the Prior Art
As is known in the art, alumina ceramic compositions have been widely used as a material for multilayer circuit boards because of the good insulating property thereof. A typical ceramic composition for this purpose is obtained by firing or sintering starting materials made, for example, of about 96% of Al.sub.2 O.sub.3 powder and the balance of SiO.sub.2, MgO, CaO and the like powders, in a reductive atmosphere at a temperature of from 1500.degree. to 1600.degree. C. For the printing of a conductor in the inside of the board, a conductive paste comprised chiefly of Mo or W has been used.
Since the alumina ceramic composition has a large coefficient of linear expansion not lower than 7.5.times.10.sup.-6 /.degree.C., it is liable to thermal shock. It has been frequently experienced that when the multilayer circuit board made of the composition is subjected to thermal shock at a difference in temperature not smaller than several degrees, cracks are inevitably produced. Accordingly, for soldering electronic parts on a multilayer wiring board, a so-called preheating process is required in which the board is preheated gently to a temperature near the melting point of a solder.